1. Field of the Invention
The present invention relates to an image forming method for use in electrophotography, electrostatic recording and electrostatic printing. In addition, the present invention also relates to an image forming apparatus and a process cartridge using the image forming method.
2. Discussion of the Background
Image forming methods in which an electrostatic latent image formed on an image bearing member (such as photoreceptors and dielectric materials) using electrophotography, electrostatic recording and electroprinting is developed with a developer including a toner have been widely used for copiers, printers and facsimile machines.
Electrophotographic image forming methods typically include the following processes:    (1) an image bearing member (such as photoreceptors) is charged with a charger (charging process);    (2) the charged image bearing member is exposed to imagewise light to form an electrostatic latent image on the image bearing member (electrostatic latent image forming process);    (3) the electrostatic latent image is developed with a developer including a charged toner to form a visual toner image on the image bearing member (developing process);    (4) the toner image is transferred onto a receiving material optionally via an intermediate transfer medium (transfer process); and    (5) the toner image on the receiving material is fixed upon application of heat and/or pressure thereto, resulting in production of a hard copy (fixing process).
When developing an electrostatic latent image with a dry developer, cascade developing methods, magnetic brush developing methods, and powder cloud developing methods have been used. These methods typically use a dry toner in which a colorant such as carbon blacks is dispersed in a binder resin.
As for the fixing method, heat fixing methods using a heat roller are widely used because of having a high energy efficiency. Recently, in order to save energy, toner images are fixed at a relatively low temperature and therefore the heat energy applied to toner images becomes smaller and smaller particularly in high speed image forming methods. Therefore, it has been attempted to perform a heat fixing operation while reducing the heating energy applied to toner images.
In addition, in order to protect environment, it is needed to decrease the warm-up time of image forming apparatus and the electric power consumption of the apparatus in a waiting state. Requirements for next generation image forming apparatuses are described in the DSM (Demand-side Management) program of IEA (International Energy Agency). There are several requirements therein such that the warm-up time should be not greater than 10 seconds and the power consumption in a waiting state should be not greater than 10 to 30 watt (which changes depending on the copying speed) in copiers having a copy speed not less than 30 cpm (copies per minutes). In order to fulfill these requirements, the power consumption of copiers must be dramatically reduced.
In attempting to fulfill the requirements, a method which uses a fixing member (such as heat rollers) having a low heat capacity or a toner having improved temperature response is proposed. However, the method is not fully satisfactory. In order to fulfill the requirements and shorten the waiting time, it is necessary to lower the fixing temperature while decreasing the minimum fixable temperature of the toner used. Specifically, it is necessary to lower the fixing temperature by about 20° C. compared to a case where a conventional toner having a low fixable temperature is used. This requirement cannot be fulfilled by a combination of conventional techniques, and it is necessary to develop an advanced technique.
When it is attempted to decrease the low temperature fixability of a toner, the fixable temperature range of the toner is typically narrowed and the high temperature. preservability of the toner deteriorates. For example, published unexamined Japanese patent applications Nos. (hereinafter referred to as JP-A) 60-90344 and 03-229264 have disclosed techniques in that a resin or a wax having a low softening point is included in a toner to improve the low temperature fixability of the toner. However, such toners cause a blocking problem in that toner particles are adhered to each other when heated in the image forming apparatus for which the toner is used or when preserved at a relatively high temperature. In addition, such toners have a narrower fixable temperature range than other conventional toners. Therefore, a toner having a wide fixable temperature range and high temperature preservability has not yet obtained.
Recently, a need for high quality copy images increases more and more. Toner having a volume average particle diameter of from 10 to 15 μm cannot fulfill this need, and therefore a need exists for a toner having a smaller particle diameter. However, as the particle diameter of toner decreases, the amount of toner particles constituting a half tone image decreases (i.e., the height of a half tone toner image decreases) and therefore the amount of heat applied to a half tone toner image formed on a recessed portion of a receiving material (such as papers) seriously decreases, resulting in occurrence of problems such as offset problems.
In addition, toner is required to have not only a good combination of electrostatic, thermal, mechanical and chemical properties but also good powder properties such as good fluidity, good blocking resistance and narrow particle diameter distribution. Therefore, toner typically includes a variety of additives.
Among the drawbacks of conventional low temperature fixable toner, the problem in that external additives of the toner which are added to improve the fluidity of the toner are embedded into or released from the toner particles is serious. The reason for occurrence of such a problem is that a binder resin having a low softening point is used for such a low temperature fixable toner. In attempting to improve the fluidity of a toner (i.e., to improve the feedability and charging properties of a toner), the following methods have been disclosed:    (1) methods in which a particulate inorganic material having an average particle diameter of from about 5 to about 100 nm is added to a toner;    (2) methods in which a particulate inorganic material having an average particle diameter of from about 0.5 to about 5 μm is added to a toner to improve cleanability of the toner (which have been disclosed in JP-As 60-136752 and 60-32060); and    (3) methods in which a particulate organic material having an average particle diameter of from about 0.05 to about 5 μm is added to a toner (which have been disclosed in JP-As 60-186854, 60-186859, 60-186864 and 60-186866).
However, toners including such a particulate inorganic material or a particulate organic material as an external additive typically cause a problem in that the particulate material is released from or embedded into toner particles as the number of produced copies increases. When such a problem occurs, the fluidity and charging property of the toner (i.e., main properties of electrophotographic toner) deteriorate. In particular, an external additive released from toner particles often scratches the surface of the photoreceptor used, resulting in occurrence of a problem in that an undesired image having a rice-fish form which is illustrated in FIG. 1 is produced. This rice-fish form image is sometimes referred to as a (white) streak image.
In attempting to solve the streak image problem, the following methods have been proposed:    (1) a method using inorganic oxides having a relatively large particle diameter of from 150 nm to 5 μm as an external additive (disclosed in published examined Japanese patent application No. 02-45188); and    (2) a method in which a silica is fixed on a surface of a toner (disclosed in JP-A02-167561).
However, the method (1) has a drawback in that the external additive is released from the toner as the number of copies increases because the adhesion of the external additive to the toner is weak. The method (2) has a drawback in that embedding of the silica into the toner particles is accelerated.
In attempting to prevent release of an external additive from toner particles, JP-A 09-96923 discloses a method in which two kinds of particulate hydrophobic metal oxides are mixed with a toner to prepare a toner, on the surface of which the particulate hydrophobic metal oxides are uniformly adhered. However, in order to fixedly adhere the metal oxides to the toner, a strong force has to be applied when the metal oxides are mixed with the toner in a mixer, and thereby the metal oxides tend to be embedded into the toner particles. Therefore, the desired effect cannot be produced.
In addition, in order to prevent formation of streak images, JP-A 2000-338718 discloses a method in which a resin including no tetrahydrofuran-insoluble components and having a specific GPC molecular weight distribution, wherein components having a molecular weight in a range from 1×105 to 1×107 have a specific dynamic viscoelasticity G′. The streak image problem cannot be fully solved by this technique when a large number (100,000 or more) of copies are produced.
On the other hand, a photoreceptor including an organic photosensitive material is typically used as an image bearing member of electrophotographic image forming apparatus. Such an organic photoreceptor is typically prepared by a method in which a charge generation layer is formed on an electroconductive substrate by depositing an organic charge generation material or coating a coating liquid including a charge generation material dispersed in a binder resin solution, and then a charge transport layer is formed on the charge generation layer by coating a coating liquid including a charge transport material dispersed in a binder resin solution.
The thus prepared organic photoreceptors have the following advantages over other photoreceptors:    (1) various materials having a sensitivity to a specific wavelength range corresponding to the light emitted by light irradiators such as visible light or infrared light have been developed;    (2) environment-friendly materials can be used; and    (3) the manufacturing cost is relatively low.
However, the organic photoreceptors have a drawback of having a low mechanical strength.
As mentioned above, a photoreceptor is typically subjected to charging, light irradiation, developing, and transferring processes. When a toner image formed on a photoreceptor is transferred on a receiving material or an intermediate transfer medium, all the particles of the toner image are not transferred, and part of the toner particles remains on the surface of the photoreceptor without being transferred. The residual toner particles are typically removed from the photoreceptor by a cleaner such as fur brushes, magnetic brushes and blades. Among these cleaners, blades made of a rubber plate are typically used as a cleaner. Thus, external forces are applied to a photoreceptor in the charging, developing, transferring and cleaning processes. Therefore, the photoreceptor has to have good durability. In particular, the photoreceptor is requested to have such a good mechanical durability as not to be damaged (scratched) even when rubbed with a charger, a developer, a cleaner or when a jammed paper is removed while rubbing the surface of the photoreceptor. In addition, as image forming apparatus are downsized, the diameter of the photoreceptors used becomes smaller and smaller. Further, a need exists for high speed and maintenance-free image forming apparatus. Therefore a need for a photoreceptor having excellent durability increases more and more.
Organic photoreceptors have a low chemical stability. In addition, photosensitive layers including a low molecular weight charge transport material and an inactive polymer as main components are generally soft. When such photosensitive layers are repeatedly used for forming images, the surface of the photoreceptors is easily abraded due to the mechanical stresses applied by developing members and cleaners. In addition, the pressure and hardness of a cleaner is increased more and more in order that toner particles having a relatively small particle diameter, which are used for forming high quality images, can be well removed from the surface of a photoreceptor. Therefore, abrasion of the surface of a photoreceptor is accelerated. When the surface of a photoreceptor is abraded, the physical properties such as photosensitivity and chargeability of the photoreceptor deteriorate, resulting in formation of undesired images such as images having low image density and/or background fouling. Further, toner particles remaining on a scratch formed on a surface of a photoreceptor cause an undesired streak image. At the present time, a photoreceptor is often replaced with a new photoreceptor before expiration of its life due to formation of a scratch thereon or serious abrasion of the surface thereof.
In attempting to fulfill the requirements mentioned above (or to solve the problems mentioned above), various proposals have been made. With respect to mechanical durability, a method in which a bisphenol Z-form polycarbonate resin is used as a binder resin of an outermost layer is proposed to improve the abrasion resistance and to prevent formation of a toner film on the surface of the outermost layer. In addition, JP-A 06-118681 discloses a photoreceptor having an outermost layer including a crosslinked silicone resin containing a colloidal silica.
However, the former technique has a drawback in that the abrasion resistance of the resultant photoreceptor is not sufficient. In contrast, the latter technique has a drawback in that the electrophotographic properties of the resultant photoreceptor are not satisfactory (i.e., background fouling and/or blurred images are formed), although the outermost layer has good abrasion resistance.
In attempting to remedy the drawbacks, photoreceptors in which a crosslinkable organic silicone polymer bonded with a silicon-modified positive hole transport compound is used for the outermost layer thereof have been proposed by JP-As 09-124943 and 09-190004. However, the photoreceptors have a drawback in that since the crosslinked outermost layer is hardly abraded, water adsorbed on the surface thereof under high humidity conditions cannot be removed, resulting in formation of blurred images. In addition, a problem in that a film of paper dust and/or toner is formed on the surface of the outermost layer, resulting in formation of undesired images such as spot images and streak images tends to occur.
Therefore, in order to prepare a photoreceptor having excellent durability, it is a serious problem to be solved in the electrophotographic field to reduce the abrasion loss of the surface of the photoreceptor. In attempting to solve the abrasion problem, the following photoreceptors have been proposed:    (1) a photoreceptor having a crosslinked charge transport layer which is prepared by crosslinking a crosslinkable binder resin (JP-A 56-48637);    (2) a photoreceptor using a charge transport polymer (JP-A 64-1728; and    (3) a photoreceptor having a crosslinked charge transport layer in which an inorganic filler is dispersed (JP-A 04-281461).
The photoreceptor (1) mentioned above has a drawback of having a high residual potential (i.e., the potential of a lighted portion of the photoreceptor is high), resulting in decrease of image density. This is because a charge transport material generally has poor compatibility with a crosslinked resin, resulting in uneven distribution of the charge transport material; and impurities included in the charge transport layer such as polymerization initiators and non-reacted functional groups increase the residual potential.
The photoreceptor (2) has a drawback of having insufficient abrasion resistance, i.e., the photoreceptor cannot fulfill the abrasion resistance requirement for photoreceptors. In addition, it is difficult to prepare a charge transport polymer having a high purity, and to control the polymerization and refinement processes in manufacturing the polymer, and thereby a charge transport layer having desired electric properties cannot be stably produced. Further, since the coating liquid typically has a high viscosity, it is difficult to stably manufacture the desired photoreceptor.
The photoreceptor (3) has better abrasion resistance than photoreceptors having a photosensitive layer in which a low molecular weight charge transport material is dispersed in an inactive polymer. However, the photoreceptor has a high residual potential because charge traps are present on the surface of particles of the inorganic filler. Therefore, the resultant images have a low image density. In addition, when the surface of the charge transport layer including an inorganic filler is rough, a cleaning problem in that toner particles remaining on the surface of the photoreceptor cannot be well removed occurs, resulting in occurrence of the toner filming problem and the blurred image problem.
Thus, the photoreceptors (1)-(3) cannot fulfill the durability requirements (i.e., a combination of electric durability and mechanical durability).
In attempting to solve the abrasion resistance and scratch resistance, Japanese Patent No. (hereinafter referred to as JP) 3,262,488 discloses a photoreceptor having a crosslinked protective layer obtained from a poly-functional acrylate monomer. Although there is a description in the patent that the protective layer can include a charge transport material, there is no specific description concerning details of the charge transport material to be included therein. As a result of the present inventors' experiment, it is found that a low molecular weight charge transport material is not well dispersed in such a crosslinked acrylic protective layer, resulting in precipitation of the charge transport material, and thereby the residual potential of the photoreceptor is increased. Therefore, the resultant photoreceptor produces low density images while having a low mechanical strength. In addition, since the protective layer is prepared by reacting the acrylic monomer in a film including a binder resin, a desired three-dimensional network cannot be formed (i.e., the density of crosslinking bonds (hereinafter crosslinking density) is low), and therefore good abrasion resistance cannot be imparted to the photoreceptor.
Further, in attempting to solve the abrasion resistance, Japanese Patent No. 3,194,392 discloses a photoreceptor having a charge transport layer which is prepared using a coating liquid including a monomer having a C═C double bond, a charge transport material having a C═C double bond and a binder resin. In this charge transport layer, the binder resin is considered to improve the adhesion of the crosslinked charge transport layer to the charge generation layer located below the charge transport layer, and to relax the internal stress of the charge transport layer when the layer is crosslinked. As the binder resin, resins having a C═C double bond, which have reactivity with the charge transport material, and other resins having no C═C double bond, which have no reactivity with the charge transport material, can be used. When a resin having no C═C double bond is used, the resin has poor compatibility with a reaction product of the monomer with the charge transport material, and thereby the charge transport layer causes a phase separation problem. Therefore, problems in that the surface of the photoreceptor is scratched, and a film of toner and/or paper dust is formed on the surface thereof occur. In addition, this charge transport layer has a low crosslinking density similarly to the case of the photoreceptor of JP 3,262,488 mentioned above. In addition, difunctional monomers are used as specific examples in JP 3,194,392. In this case, the resultant photoreceptor has insufficient abrasion resistance. In addition, when a binder having a reactivity is used and the molecular weight of the crosslinked layer increases, the layer has low crosslinking density. Namely, it is difficult to prepare a layer having-a combination of high charge transport material bonding density and high crosslinking density. Therefore, it is difficult to impart a good combination of electric properties and abrasion resistance to the photoreceptor.
JP-A 2000-66425 discloses a photoreceptor having a photosensitive layer which is prepared by crosslinking a positive hole transport compound having two or more chain polymerization functional groups in a molecule. The photosensitive layer has a high hardness because of having a high crosslinking density. However, since the positive hole transport compound is bulky and has two or more chain polymerization functional groups, the crosslinked layer tends to be distorted, resulting in increase of stress in the layer. Therefore, a problem in that the crosslinked layer causes cracks and/or peeling occurs when the photoreceptor is repeatedly used for a long period of time.
Thus, the above-mentioned photoreceptors having a crosslinked charge transport layer having a charge transport structure are not satisfactory.
JP-A 2002-251032 discloses a photoreceptor which hardly causes the streak image problem even when a low temperature fixable toner is used in combination therewith. The photoreceptor has an outermost layer including a particulate alumina (i.e., an inorganic filler) and a tetrafluoroethylene powder (i.e., an organic filler) to improve the mechanical strength of the surface of the photoreceptor and to decrease the friction coefficient of the surface. However, when an inorganic filler is included in an outermost layer, a problem in that a silica, which is used as an external additive and which is hard, sticks in a binder resin, which is located between a filler particle and another filler particle in the surface portion of the outermost layer and which is soft. This often results in occurrence of the streak image problem. When the filler content of the outermost layer is increased to solve the problem, the layer becomes brittle, resulting in deterioration of the mechanical properties of the layer. In addition, the layer has poor electric properties because of having a high residual potential due to the inorganic filler. In this case, when the photoreceptor is repeatedly used, the filler particles in the surface portion are exposed without covered with a binder resin and thereby a problem in that undesired black spot images are produced is caused.
As mentioned above, a photoreceptor having a good combination of low temperature fixability and durability (i.e., electric durability, mechanical durability and chemical durabililty) has not yet developed. Therefore, an image forming apparatus (or method) which fulfills the energy saving requirement and which is so durable as to be able to produce images with hardly replacing the image bearing member is not yet developed.